README.md: fix link to `test_index_racy__diff`

The syntax for links is `[description](link)z, not the other way round.
Fix this.

README.md

@@ -217,7 +217,7 @@ run the index tests:
     $ ./libgit2_clar -sindex
 
 To run a single test named `index::racy::diff`, which corresponds to the test
-function (`test_index_racy__diff`)[https://github.com/libgit2/libgit2/blob/master/tests/index/racy.c#L23]:
+function [`test_index_racy__diff`](https://github.com/libgit2/libgit2/blob/master/tests/index/racy.c#L23):
 
     $ ./libgit2_clar -sindex::racy::diff